The present invention relates to an amplifier circuit, for use in an optical disk drive, for converting a photoelectric current into a voltage.
Today, a variety of media types of optical disks, such as CDs (compact disks) and DVDs (digital versatile disks), have been practically used. The CDs can be roughly classified into read only disks and writable disks. The read only disks include CD-DAs for digital audio, CD-ROMs mainly used for recording computer data and the like. The writable disks include CD-Rs that are once writable, CD-RWs that are writable and erasable and the like. The DVDs can be also roughly classified into read only disks and writable disks. The read only DVDs include DVD-ROMs and the like, and the writable DVDs include DVD-Rs that are once writable, DVD-RAMs that are writable and erasable and the like. In consideration of such a situation, a recent optical disk drive has become applicable to a plurality of media types.
In general, an optical disk drive irradiates an optical disk loaded therein with a laser beam so as to reproduce data by detecting reflected light. For example, an infrared laser beam is used for a CD and a red laser beam is used for a DVD. A light receiving unit of the optical disk drive includes a photodiode for generating a photoelectric current in accordance with the reflected light from the optical disk, and the photoelectric current is converted into a voltage by an amplifier circuit (i.e., a current-voltage conversion circuit).
Japanese Laid-Open Patent Publication No. 10-340526 discloses a technique to discriminate a media type of an optical disk by detecting reflectance of the optical disk on the basis of a peak level of a focus error signal. This publication further discloses an optical disk drive that selects any of a plurality of amplifier circuits having different gains or controls a power of the laser beam in accordance with the discriminated media type. Specifically, for example, when a CD-DA/ROM and a CD-RW are compared with each other, the reflectance of the CD-RW is as low as approximately ¼ of that of the CD-DA/ROM due to their difference in recording principle. Accordingly, in reproducing data from a CD-RW, an amplifier circuit with a large gain is selected or the power of the laser beam is increased, so that an output voltage level can be constant regardless of media type.
Japanese Laid-Open Patent Publication No. 2000-293933 discloses an optical disk drive that can control a gain of a single amplifier circuit on the basis of a discriminated media type. Specifically, for a DVD-RAM with low reflectance, a power of the laser beam is increased as compared with that for a DVD-ROM and a resistor with a large resistance value is selected as a feedback resistor of an operational amplifier included in the amplifier circuit, so that the gain of the amplifier circuit can be enhanced. Thus, an output level of the amplifier circuit can be constant regardless of media type.
On the other hand, a reproducing speed for DVD-ROMs is increasing. For example, for attaining 16× reproducing speed as compared with standard one, an amplifier circuit is required to have a frequency band as wide as 72 MHz (1 dB down) for DVD-ROMs. A recording/reproducing speed for DVD-RAMs is not demanded to be as high as the reproducing speed for DVD-ROMs, but there is a strong demand for lowering noise in recording/reproducing of DVD-RAMs.
However, a large bias current is conventionally supplied commonly to a pair of differential input transistors of an operational amplifier included in an amplifier circuit for converting a photoelectric current into a voltage, so as to uniformly attain a wide band characteristic regardless of media type. Accordingly, shot noise is caused in each of the pair of differential input transistors to the same extent in loading a DVD-ROM as in loading a DVD-RAM. Moreover, since a gain of the amplifier circuit is enhanced in loading a DVD-RAM, an output noise is increased. Also, this means that power is wastefully consumed in loading a DVD-RAM.